#include "random.h"
#include <stdbool.h>
#include <stdint.h>
#include "debug.h"

/* RC4-based pseudo-random number generator (PRNG).

   RC4 is a stream cipher.  We're not using it here for its
   cryptographic properties, but because it is easy to implement
   and its output is plenty random for non-cryptographic
   purposes.

   See http://en.wikipedia.org/wiki/RC4_(cipher) for information
   on RC4.*/

/* RC4 state. */
static uint8_t s[256];   /* S[]. */
static uint8_t s_i, s_j; /* i, j. */

/* Already initialized? */
static bool inited;

/* Swaps the bytes pointed to by A and B. */
static inline void swap_byte(uint8_t *a, uint8_t *b)
{
    uint8_t t = *a;
    *a = *b;
    *b = t;
}

/* Initializes or reinitializes the PRNG with the given SEED. */
void random_init(unsigned seed)
{
    uint8_t *seedp = (uint8_t *)&seed;
    int i;
    uint8_t j;

    for (i = 0; i < 256; i++)
        s[i] = i;
    for (i = j = 0; i < 256; i++)
    {
        j += s[i] + seedp[i % sizeof seed];
        swap_byte(s + i, s + j);
    }

    s_i = s_j = 0;
    inited = true;
}

/* Writes SIZE random bytes into BUF. */
void random_bytes(void *buf_, size_t size)
{
    uint8_t *buf;

    if (!inited)
        random_init(0);

    for (buf = buf_; size-- > 0; buf++)
    {
        uint8_t s_k;

        s_i++;
        s_j += s[s_i];
        swap_byte(s + s_i, s + s_j);

        s_k = s[s_i] + s[s_j];
        *buf = s[s_k];
    }
}

/* Returns a pseudo-random unsigned long.
   Use random_ulong() % n to obtain a random number in the range
   0...n (exclusive). */
unsigned long
random_ulong(void)
{
    unsigned long ul;
    random_bytes(&ul, sizeof ul);
    return ul;
}
